The present invention relates generally to computing, or microprocessor-based equipment and is particularly directed to a segmented spectrum clock generator of the type which reduces electromagnetic interference (EMI) emissions. The invention is specifically disclosed as a segmented spectrum clock generator that divides the EMI spectrum into multiple low bandwidth components having sharp roll-offs to reduce EMI emissions.
High-speed digitally clocked systems are typically very noisy with regard to electromagnetic interference (EMI) emissions, unless some special care is taken at the design stage of equipment incorporating such clocked systems. One reliable and low-cost method for reducing EMI emissions is to use a spread spectrum clock such as is disclosed in U.S. Pat. Nos. 5,488,627 and 5,631,920. These patents disclose circuits in which the spread spectrum frequencies are varied by the use of programmable counters and by data stored in a memory circuit. These U.S. Pat. Nos. 5,488,627 and 5,631,920 are assigned to Lexmark International, Inc.
In a U.S. Pat. No. 6,167,103 (filed Oct. 8, 1998), a digital spread spectrum clock circuit is disclosed in which the clock is made variable by using Random Access Memory and a multiplexer to receive initiation data before the clock circuit is ready to run normally. This patent is titled xe2x80x9cVariable Spread Spectrum Clock,xe2x80x9d is commonly-assigned to Lexmark International, Inc.
In a U.S. Pat. No. 6,292,507 (filed Sep. 1, 1999), a method for automatically compensating a spread spectrum clock generator circuit is disclosed that measures the pulse-width of the phase locked loop UP and DOWN signals and compares their actual pulse widths to predetermined values and corrects any deviation. This patent is titled xe2x80x9cMethod and Apparatus for Compensating a Spread Spectrum Clock Generator,xe2x80x9d and is assigned to Lexmark International, Inc.
While such prior spread spectrum clocks have often been disclosed or constructed using phase locked loop circuits, other types of frequency synthesizer circuits can be made into a spread spectrum clock, including digital locked loop circuits and delay locked loop circuits. One example of a digital locked loop circuit is disclosed in U.S. Pat. No. 5,079,519, and one example of a delay locked loop circuit is disclosed in U.S. Pat. No. 5,771,264. U.S. Pat. Nos. 6,046,646 and 5,610,955 disclose other ways to create a spread spectrum clock.
The spread spectrum clock generator designs previously available have a design sensitivity to the voltage controlled oscillator gain, charge pump current, and passive component values (in connection with phase locked loop circuits). Moreover, previous spread spectrum clock generator circuits would likely not meet certain EMI tests that have been proposed in a white paper, recently published on Mar. 18, 2000 by the Radiocommunications Agency of the United Kingdom, under RA Ref. No. AY3377(510001891), as a proposed amendment to the CISPR-22 rules.
In the CISPR-22 proposed rules, the regulations for xe2x80x9cbroadbandxe2x80x9d emissions from electronic products would be changed such that the use of a spread spectrum clock generator circuit would be required to meet much more stringent emissions requirements, and would potentially cancel most of the benefits of the use of a spread spectrum clock. The proposed rules perform an emissions test on an emitted signal, and if the signal has certain characteristics it is considered a broadband signal and would be subject to new emissions limits. If the emitted signal does not have those certain characteristics, it would then be classified as a narrowband signal and would remain subject to the existing emissions limits.
The proposed test would determine if a particular emitted frequency has an amplitude that is within 10 dB of the current limits, and if so, then it is tested to determine if it has broadband or narrowband characteristics. The proposed test measures the emissions at xc2x1150 kHz of the frequency of interest. If these two bracketing or offset frequencies (i.e., at xc2x1150 kHz) measure within 10 dB of the current limits, then the original signal under test is considered a broadband signal. If that is the case, then the allowed emissions limits are much lower than before.
The new measurement method being proposed in this report is as follows:
For each and every disturbance above (Lxe2x88x9210 dB), where L is the limit level in logarithmic units, record the frequency fn at which the maximum disturbance level occurs and the disturbance level dn at that frequency. Record the antenna polarization for each reported disturbance. For each disturbance above (Lxe2x88x9210 dB), measure the level of the disturbance dnh at (fn+150 kHz) and dnl at (fnxe2x88x92150 kHz). If dnh and dnl are both 10 dB or more below the limit level, the disturbance shall be regarded as a narrow band disturbance and the limits of tables 3 or 4 shall apply. If either dnh or dnl are less than 10 dB below the limit level, the disturbance shall be regarded as a broad band disturbance and the following procedure shall be followed.
The frequency of the measuring receiver shall be adjusted upwards from fn in increments not exceeding 100 kHz until a frequency fnh is reached where (Lxe2x88x92dnh) greater than 10 dB. The frequency of the measuring receiver shall then be adjusted downwards from fn in increments not exceeding 100 kHz until a frequency fnl is reached where (Lxe2x88x92dnl) greater than 10 dB. The total bandwidth ≅f occupied by the emission shall be calculated from ≅f=(fnhxe2x88x92fnl). The level of each individual broad band emission at frequencies fn, fn+l, etc. shall not exceed (Lxe2x88x9210 Log10(≅f/120 kHz)*) or (Lxe2x88x9210 dB*) whichever is the greater, where L is the applicable limit level in logarithmic units from table 3 or 4.
Conventional single frequency clock circuits with high jitter or conventional spread spectrum clock generation circuits will likely have problems meeting the proposed new standard if it becomes implemented in Europe. A new type of clock is needed to satisfy the proposed new rules of CISPR-22, since a standard spread spectrum clock generation circuit creates a relatively flat bandwidth in the portion of the frequency spectrum where it resides. Under the new CISPR-22 proposed test this would be classified as a broadband emission. To be considered a narrowband emission, the frequency spectrum produced by the clock circuit must have frequent peaks and valleys to chop or xe2x80x9csegmentxe2x80x9d the spectrum profile.
Accordingly, it is an advantage of the present invention to provide a segmented spectrum clock generator that creates a spectrum shape in the frequency domain that has multiple peaks and valleys within the frequency ranges of interest. It is another advantage of the present invention to provide a segmented spectrum clock generator that emits multiple peaks and valleys that occur at appropriate intervals so that the roll-off points to both sides of each peak are below 10 dB at the bracketing (offset) frequencies that are at intervals of +150 kHz and xe2x88x92150 kHz of the peak frequency. It is another advantage of the present invention to provide a segmented spectrum clock generator that provides a segmented spectrum form in the frequency domain having multiple peaks and valleys, and does so with a frequency synthesizer circuit at the heart of the electronics, in which the frequency synthesizer circuit could be a phase locked loop, a digital lock loop, a delay locked loop, or some other similar circuitry. It is still another advantage of the present invention to provide a segmented spectrum clock generator that produces a spectrum shape in the frequency domain that has a segmented set of multiple peaks and valleys, and is based upon a frequency synthesizer circuit that is flexible in design such that it could also be used as a spread spectrum clock generator circuit by changing certain settings that either are programmable, or can be determined at the time of manufacture. It is a further advantage of the present invention to provide a segmented spectrum clock generator that modulates its operating frequency at a predetermined modulation profile, and which determines a lower bound and an upper bound for the modulation frequency.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a method for controlling a segmented spectrum clock generator circuit provides a clock signal to a frequency synthesizer circuit, and provides a controller; wherein the frequency synthesizer under control of the controller generates a plurality of output frequencies over time; and the plurality of output frequencies are modulated over at least one time interval according to a modulation profile such that a frequency response of the plurality of output frequencies exhibits a segmented waveform over a spectrum of amplitude versus frequency, in that the segmented waveform comprises a plurality of individual segments that each exhibit: (a) a maximum amplitude and (b) a pair of minima amplitudes, wherein a slope of the frequency response between the maximum amplitude and each of the pair of minima amplitudes exhibits a 10 dB rolloff at a predetermined frequency difference from a center frequency of the individual segment.
In accordance with another aspect of the present invention, a method for controlling a segmented spectrum clock generator circuit at a modulation frequency above a lower bound provides a clock signal to a frequency synthesizer circuit, and provides a controller, in which the frequency synthesizer under control of the controller generates a plurality of output frequencies over time; and the plurality of output frequencies are modulated over at least one time interval according to a modulation profile, such that a frequency response of the plurality of output frequencies exhibits a segmented waveform over a spectrum of amplitude versus frequency, in that the segmented waveform comprises a plurality of individual segments that each exhibit: (a) a maximum amplitude and (b) a pair of minima amplitudes, wherein the plurality of output frequencies is modulated at a rate greater than a lower bound that is dependent upon: a predetermined offset value, threshold value, and harmonic separation, and which takes into account pass-band limit of overall selectivity of a receiver used to test the segmented spectrum clock generator.
In accordance with a further aspect of the present invention, a method for controlling a segmented spectrum clock generator circuit at a modulation frequency below an upper bound provides a clock signal to a frequency synthesizer circuit, and provides a controller, in which the frequency synthesizer under control of the controller generates a plurality of output frequencies over time; and the plurality of output frequencies are modulated over at least one time interval according to a modulation profile, such that a frequency response of the plurality of output frequencies exhibits a segmented waveform over a spectrum of amplitude versus frequency, in that the segmented waveform comprises a plurality of individual segments that each exhibit: (a) a maximum amplitude and (b) a pair of minima amplitudes, wherein the plurality of output frequencies is modulated at a rate less than an upper bound that is dependent upon: clock frequency, frequency deviation, and a predetermined clock harmonic where attenuation is desired.
In accordance with still a further aspect of the present invention, a segmented spectrum clock generator circuit is provided comprising a controller and a frequency synthesizer circuit having a clock signal input, the frequency synthesizer under control of the controller generating a plurality of output frequencies over time according to a modulation profile, such that a frequency response of the plurality of output frequencies exhibits a segmented waveform over a spectrum of amplitude vs. frequency, in that the segmented waveform comprises a plurality of individual segments that each exhibit: (a) a maximum amplitude and (b) a pair of minima amplitudes, wherein a slope of the frequency response between the maximum amplitude and each of the pair of minima amplitudes exhibits a 10 dB rolloff at a predetermined frequency difference from a center frequency of the individual segment.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.